1. Field of the Invention
The present invention relates to an ink supply apparatus and an ink jet recording apparatus.
2. Description of the Related Art
Japanese Patent Application Laid-Open No. 2002-307709 (corresponding to U.S. Pat. No. 6,805,437) discusses an ink supply system of an ink jet recording apparatus, which is an example of the conventional ink jet recording apparatus shown in FIG. 4.
A recording head 101 has a sub tank 101b. The sub tank 101b holds a predetermined amount of ink supplied through a tube 106 from a main tank 104 mounted on the main body of the ink jet recording apparatus.
The main tank 104 is detachably mounted on a supply unit 105, which is provided with an ink supply port sealed by a rubber stopper 104b and an atmosphere introduction port sealed by a rubber stopper 104c, at its base portion. The main tank 104 is an air-tight container as a unit. The ink 109 is directly contained inside the main tank 104 without being immersed into an ink absorber.
The ink supply unit 105 includes an ink supply needle 105a and an air introducing needle 105b. The ink supply needle 105a draws out the ink 109 from the main tank 104 and supplies the ink to the recording head 101. The air introducing needle 105b introduces air from an air communication port 105g into the main tank 104. The main tank 104 is attached to the ink supply unit 105, and the ink introducing needle 105a and the air introducing needle 105b penetrate the rubber stoppers 104b and 104c to enter an interior of the main tank 104. That is, the main tank 104 is attached to the ink supply unit 105 so that a liquid path from the recording head 101 to the ink supply needle 105a, and a liquid path from the air introducing needle 105b to the atmosphere communication port 105g are connected as a single liquid path. The ink is supplied from a connector inserting slot 101a to the recording head 101. A liquid connector provided at an end of the ink supply tube 106 is connected airtight to the connector inserting slot 101a. 
In the above-described head and the ink supply system, when the air in the recording head 101 and the main tank 104 expands due to change in the ambient temperature, it is necessary to prevent the ink from flowing out from the atmosphere communication port 105g. Accordingly, the atmosphere communication chamber 105f is arranged between the air introducing needle 105b and the atmosphere communication port 105g so that the overflowing ink can be temporarily stored therein.
A liquid path 105k from the atmosphere communication chamber 105f to the atmosphere communication port 105g has a predetermined cross-sectional area. A portion of the liquid path is positioned higher than the port of the air introducing needle 105b in order to prevent the ink inside the atmosphere communication chamber 105f from leaking outside when the apparatus is inclined to some extent.
In the above-described configuration, when the ink inside the recording head 101 is consumed, the ink is supplied from the main tank 104 to the recording head 101 through the ink supply unit 105 and the ink supply tube 106, due to the negative pressure. At this time, the same amount of air as the ink supplied from the main tank 104 is introduced into the main tank 104 from the atmosphere communication port 105g, through the atmosphere communication chamber 105f and the air introducing needle 105b. 
The air permeating and intruding through a resinous material of the tube 106, or the air dissolved and contained inside the ink can cause air accumulation inside the sub tank 101b, which causes a problem. Moreover, the ink supply can fail when the ink inside the sub tank 101b becomes empty.
The accumulated air is removed by a sequential operation of a shut-off valve 110 situated at a flow path from the recording head 101 to the ink supply needle 105a, in association with a recovery unit 107 that includes a suction cap 107a that sucks the discharge nozzles 101g, and the suction pump 107b. 
However, the conventional example described above has a problem that the ink may leak not from the main tank 104 side but from the recording head 101 side. In this state where the ink of the main tank 104 flows into the atmosphere communication chamber 105f due to an increase in the ambient temperature, if the recording head 101 is forcibly removed or the ink supply tube 106 is cut, the following problems occur: after the atmosphere communication chamber 105f is filled with the ink, once the ink starts to flow outside from the liquid path 105k that connects the atmosphere communication chamber 105f to the atmosphere, a siphon principle comes to act. When the siphon principle acts, a tube saturated with the ink of the main tank 104 creates a continuous ink flow until all ink inside the main tank 104 is empty. According to the siphon principle, when a fluid flows down the tube in a saturated condition, a suction effect occurs at an upstream side of the fluid flow. Moreover, once the apparatus is inclined, meniscus of the recording head is broken. As a result, the ink fills up the entire liquid path 105k that connects the atmosphere communication chamber 105f to the outside air, and the ink is discharged from the atmosphere communication port 105g to the outside.
The negative pressure inside the main tank 104 increases, and the same amount of ink as that flowed out to the outside, flows into the main tank 104 via the tube 106 from the recording head in which meniscus is broken. If the ink continues to be supplied from the main tank 104 to the air introducing needle 105b, the ink continues to flow out from the atmosphere communication port 105g to the outside. As a result, the ink inside the main tank 104 becomes empty.